Because Acrobat XI is EOL, any vulnerabilities discovered after October 2017 remain unpatched. Version 11.0.23 was the final update, but it contains numerous known security holes that have been exploited in the wild. Malicious actors often use malicious PDF files to execute code on unpatched Acrobat readers.
If you have an earlier version of Acrobat XI (like 11.0.0), you must apply the 11.0.23 patch manually, as the built-in "Check for Updates" feature may no longer connect to active servers. FULL Adobe Acrobat XI Pro 11.0.23 Patch